Mass spectrometers are widely used to separate and analyse ions on the basis of their mass to charge ratio (m/z) and many different types of mass spectrometer are known. Whilst the present invention has been designed with Time-of-flight (TOF) mass spectrometry in mind and will be described for the purpose of illustration with TOF mass spectrometry, the invention is applicable to other types of mass spectrometry. Herein ions will be referred to as an example of charged particles without excluding other types of charged particles unless the context requires it.
Time-of-flight (TOF) mass spectrometers determine the mass to charge ratio (m/z) of ions on the basis of their flight time along a fixed flight path. The ions are emitted from a pulsed source in the form of a short packet of ions, and are directed along the fixed flight path through an evacuated region to an ion detector. A packet of ions comprises a group of ions, the group usually comprising a variety of mass to charge ratios, which is, at least initially, spatially confined.
The ions leaving the pulsed source with a constant kinetic energy reach the detector after a time which depends upon their mass, more massive ions being slower. A TOF mass spectrometer requires an ion detector with, amongst other properties, fast response time and high dynamic range, i.e. the ability to detect both small and large ion currents including quickly switching between the two, preferably without problems such as detector output saturation. Such a detector should also not be unduly complicated in order to reduce cost and problems with operation.
An existing approach to dynamic range uses the output of one detector which is amplified at two different levels, e.g. as described in GB 2457112 A. This amplification is carried out either within the electron multiplication device or in the preamplifier stage. These two amplified outputs from the same detector are then used to produce a high dynamic range spectrum. Other proposed solutions to the problem of detector dynamic range in TOF mass spectrometry have included the use of two collection electrodes of different surface areas for collecting the secondary electrons emitted from an electron multiplier (U.S. Pat. Nos. 4,691,160, 6,229,142, 6,756,587 and 6,646,252) and the use of electrical potentials or magnetic fields in the vicinity of anodes to alter so-called anode fractions (U.S. Pat. No. 6,646,252 and US 2004/0227070 A). Another solution has been to use two or more separate and completely independent detection systems for detection of secondary electrons produced from incident particles (U.S. Pat. No. 7,265,346). A further solution has been the use of an intermediate detector located in the TOF separation region which provides feedback to control gain of the final electron detector (U.S. Pat. No. 6,674,068). The problem with the latter detection is that it requires fast change of gain on the detector and it is also difficult to keep track of the gain in order to maintain linearity. A still further detection arrangement proposed in US2004/0149900A utilises a beam splitter to divide a beam of ions into two unequal portions which are detected by separate detectors. In all, these detection solutions can be complicated and costly to implement and/or their sensitivity and/or their dynamic range can be lower than desired.
Another problem with TOF mass spectrometers is that they also produce data at a very high rate since the detector output comprises a large number of ion detection signals in sequence within a very short interval of time, e.g. an entire TOF mass spectrum may be detected within a few milliseconds with a data sampling rate of, for example, 1 GHz or higher. Furthermore, many spectra, for example up to one million spectra or more, may be required for a given sample to be analyzed. Improvements in the acquisition and processing of data from a TOF mass spectrometer are therefore also desirable, e.g. methods to reduce the amount of data for processing as well as the duration and efficiency of data processing.
WO 2008/08867 describes the use of microprocessors and field programmable gate arrays (FPGAs) for the application of mathematical transformations to the output of ion detectors. For high speed applications, the spectra are thus at least pre-processed on the fly. Using mathematical transformations producing mass-intensity pairs in the FPGA which are then transferred to a computer is described in U.S. Pat. No. 6,870,156. Such methods use one detector which is amplified at two different levels as described above to provide two different gain signals to which the mathematical transformations are applied. A method for reducing the data set is described in U.S. Pat. No. 5,995,989, which comprises use of a background noise threshold which is continually determined and used to filter the data and decide which data to keep for subsequent processing. The application of the threshold in that method therefore involves continual calculation.
A further method for the measurement of ions by coupling different measurement methods is disclosed in U.S. Pat. No. 7,220,970, in which a collector and an SEM are used, the ions being selectively delivered to the collector or the SEM. In U.S. Pat. No. 7,238,936 is described a means to adjust detector gain in non-TOF spectrometers where there is sufficient time for an intermediate stage of detection to disable a subsequent stage of detection.
Accordingly, there remains a need to improve the detection of ions in mass spectrometry and in particular data acquisition systems and methods. In view of the above background, the present invention has been made.